U.S. patent number 4,529,271 [Application Number 06/357,602] was granted by the patent office on 1985-07-16 for matrix addressed bistable liquid crystal display.
This patent grant is currently assigned to AT&T Bell Laboratories. Invention is credited to Dwight W. Berreman, William R. Heffner, Allan R. Kmetz.
United States Patent |
4,529,271 |
Berreman , et al. |
July 16, 1985 |
**Please see images for:
( Certificate of Correction ) ** |
Matrix addressed bistable liquid crystal display
Abstract
A matrix addressed, bistable liquid crystal optical display is
disclosed. The display includes a liquid crystal twist cell which
has at least two states which are stable in the presence of a
single given holding voltage. A form of 3:1 matrix addressing is
used in the display which enhances operational characteristics.
Inventors: |
Berreman; Dwight W. (New
Providence, NJ), Heffner; William R. (Plainfield, NJ),
Kmetz; Allan R. (Chatham, NJ) |
Assignee: |
AT&T Bell Laboratories
(Murray Hill, NJ)
|
Family
ID: |
23406291 |
Appl.
No.: |
06/357,602 |
Filed: |
March 12, 1982 |
Current U.S.
Class: |
349/34;
349/175 |
Current CPC
Class: |
G02F
1/1391 (20130101); G09G 3/3629 (20130101); G09G
2300/0486 (20130101); G09G 2320/0209 (20130101); G09G
2310/061 (20130101); G09G 2310/063 (20130101); G09G
2310/06 (20130101) |
Current International
Class: |
G02F
1/139 (20060101); G02F 1/13 (20060101); G09G
3/36 (20060101); G02F 001/13 () |
Field of
Search: |
;350/331R,333,334,341,349,35R |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Cheng, J. et al., "Switching Characteristics and Threshold
Properties of Electrically-Switched Nematic Liquid Crystal Bistable
Configuration Devices," 1980 IEEE Biennial Display Research
Conference, (Oct. 1980), pp. 180-182. .
Thurston, R. N. et al., "Mechanically Bistable Liquid-Crystal
Display Structures," IEEE Transactions on Electron Devices, vol.
ED-27, No. 11, (Nov. 1980), pp. 2069-2080..
|
Primary Examiner: Corbin; John K.
Assistant Examiner: Gallivan; Richard F.
Attorney, Agent or Firm: Tiegerman; Bernard Schneider; Bruce
S.
Claims
What is claimed is:
1. A liquid crystal optical display, comprising:
a cholesteric liquid crystal;
two bounding surfaces which confine the liquid crystal, at least
one of which bounding surfaces transmits electromagnetic
radiation;
voltage means for applying voltage differences across one or more
of a plurality of discrete portions of the liquid crystal, which
discrete portions are arranged in a matrix of rows and columns;
and
means for optically discriminating between two different
orientations of the liquid crystal, characterized in that
said liquid crystal assumes a helical configuration in an
unstrained state, the liquid crystal at at least one of the
bounding surfaces is obliquely inclined with respect to said
bounding surface, and the liquid crystal has at least two stable
states when in the presence of the single holding voltage, which
stable states are capable of being switched from one to the other
without passing a disclination through the liquid crystal, and
said voltage means substantially simultaneously switches all said
discrete portions of at least one row or column of said matrix from
a first stable state to a second stable state by applying to all
said discrete portions in said row or column a voltage below said
holding voltage for a first preselected length of time, and then
selectively switches discrete portions of said matrix from said
second stable state to said first stable state by applying to
selected discrete portions a voltage above said holding voltage for
a second preselected length of time, thereby causing information to
be displayed.
2. The display of claim 1 wherein said voltage means switches a
selected discrete portion of said liquid crystal from said second
stable state to said first stable state by applying to said
selected discrete portion, for said second preselected length of
time, a voltage substantially equal to three times the holding
voltage while nonselected discrete portions are subjected to a
voltage substantially equal to the holding voltage.
3. The display of claim 1 wherein said holding voltage takes on any
value within a range of voltages determined by the properties of
the liquid crystal optical display, said range described by V.sub.H
.+-..DELTA.V, V.sub.H being a mean voltage value of said range and
.DELTA.V being a voltage increment above and below V.sub.H
describing the limits of said range.
4. The display of claim 3 wherein said voltage means includes a
plurality of electrodes arranged on each of said bounding surfaces,
the electrodes on one bounding surface being arranged transversely
with respect to the electrodes on the other bounding surface, the
projections of the electrodes on one bounding surface onto the
electrodes of the other bounding surface defining said matrix of
discrete portions of said liquid crystal, and the electrodes on
each of said bounding surfaces constituting, respectively, row and
column electrodes which correspond to the rows and columns of said
matrix.
5. The display of claim 4 wherein said voltage means switches
selected discrete portions from said second stable state to said
first stable state by applying, in sequence, a voltage
-(1+.DELTA.z)V.sub.H to each row electrode while all other row
electrodes receive a voltage (1+.DELTA.y)V.sub.H, and substantially
simultaneously applying a voltage (2+.DELTA.x)V.sub.H to each
column electrode which defines a selected discrete portion with the
row electrode receiving the voltage-(1+.DELTA.z)V.sub.H and zero
voltage to all other column electrodes, said voltages being
supplied for said second preselected length of time, and .DELTA.x,
.DELTA.y, and .DELTA.z taking on the values
.vertline..DELTA.y.vertline..ltoreq..DELTA.v/V.sub.H,
.vertline..DELTA.x-.DELTA.y.vertline..ltoreq..DELTA.V/V.sub.H,
and the magnitude of .DELTA.z is such that any discrete portion
remains stable when subjected to a voltage V.sub.H (1+.DELTA.z)
applied for said second preselected length of time.
6. Apparatus in accordance with claim 5 wherein
.vertline..DELTA.z.vertline..ltoreq..DELTA.V/V.sub.H.
7. The display of claim 4 wherein said voltage means switches
selected discrete portions from said second stable state to said
first stable state by applying, in sequence, a voltage
(2+.DELTA.x)V.sub.H to each column electrode while all other column
electrodes receive zero voltage, and substantially simultaneously
applying a voltage -(1+.DELTA.z)V.sub.H to the row electrodes
defining a selected discrete portion with the column electrode
receiving the voltage (2+.DELTA.x)V.sub.H and a voltage
(1+.DELTA.y)V.sub.H to all other row electrodes, said voltages
being applied for said second preselected length of time, and
.DELTA.x, .DELTA.y, .DELTA.Z taking on the values
.vertline..DELTA.y.vertline..ltoreq..DELTA.V/V.sub.H,
.vertline..DELTA.z.vertline..ltoreq..DELTA.V/V.sub.H,
and the magnitude of .DELTA.x is such that any discrete portion
remains stable when subjected to a voltage V.sub.H
(1+.DELTA.x-.DELTA.y) applied for said second preselected length of
time.
8. The display of claim 7 wherein
.vertline..DELTA.x-.DELTA.y.vertline..ltoreq..DELTA.V/V.sub.H.
9. A method for displaying information, comprising the step of
changing the spatial orientations of discrete portions of the
cholesteric liquid crystal of a liquid crystal optical display
which includes bounding surfaces which confine the cholesteric
liquid crystal, at least one of which bounding surfaces transmits
electromagnetic radiation, and which includes means for optically
discriminating between at least two different orientations of the
liquid crystal, and wherein the discrete portions of the liquid
crystal are arranged in a matrix of rows and columns, characterized
in that
said liquid crystal assumes a helical configuration in an
unstrained state, the liquid crystal at at least one of the
bounding surfaces is inclined obliquely with respect to the
bounding surface, and the liquid crystal has at least two stable
states when in the presence of a single, nonzero holding voltage,
which stable states may be switched from one to the other without
passing a disclination through the liquid crystal, and
said changing step includes the steps of nonselectively and
substantially simultaneously switching all the discrete portions of
at least one row or one column of said matrix from a first stable
state to a second stable state by applying to all said discrete
portions in said row or column a voltage below said holding voltage
for a first preselected length of time, and then selectively
switching discrete portions of said matrix from said second stable
state to said first stable state by applying to selected discrete
portions a voltage above said holding voltage for a second
preselected length of time.
10. The method of claim 9 wherein said selective switching step
includes the step of applying a voltage substantially equal to
three times the holding voltage to the selected discrete portions
for said second preselected length of time while applying a voltage
substantially equal to the holding voltage to nonselected discrete
portions.
11. The method of claim 9 wherein said holding voltage takes on any
value within a range of voltages determined by the properties of
the liquid crystal optical display, said range described by V.sub.H
.+-..DELTA.V, V.sub.H being a mean voltage value of said range and
.DELTA.V being a voltage increment above and below V.sub.H
describing the limits of said range.
12. The method of claim 11 wherein said selective switching step
includes the steps of:
sequentially applying a voltage -(1+.DELTA.z)V.sub.H to each row of
discrete portions of said matrix while applying a voltage
(1+.DELTA.y)V.sub.H to all other rows of discrete portions of said
matrix; and
substantially simultaneously applying a voltage (2+.DELTA.x)V.sub.H
to each column of discrete portions which contains a selected
discrete portion lying within said row of discrete portions
receiving the voltage-(1+.DELTA.z)V.sub.H and zero voltage to all
other columns of discrete portions, said voltages being applied for
said second preselected length of time, and .DELTA.x, .DELTA.y and
.DELTA.z take on the values
.vertline..DELTA.y.vertline..ltoreq..DELTA.V/V.sub.H,
.vertline..DELTA.x-.DELTA.y.vertline..ltoreq..DELTA.V/V.sub.H,
and the magnitude of .DELTA.z is such that any discrete portion
remains stable when subjected to a voltage V.sub.H (1+.DELTA.z)
applied for said second preselected length of time.
13. The method of claim 12 wherein
.vertline..DELTA.z.vertline..ltoreq..DELTA.V/V.sub.H.
14. The method of claim 11 wherein said selective switching step
includes the steps of:
sequentially applying a voltage (2+.DELTA.x)V.sub.H to each column
of discrete portions of said matrix while all other columns of
discrete portions receive zero voltage; and
substantially simultaneously applying a voltage
-(1+.DELTA.z)V.sub.H to the rows of discrete portions which contain
selected discrete portions lying within said column of discrete
portions receiving the voltage (2+.DELTA.x)V.sub.H and a voltage
(1+.DELTA.y)V.sub.H to all other rows of discrete portions, said
voltages being applied for said second preselected length of time,
and .DELTA.x, .DELTA.y, .DELTA.z taking on the values
.vertline..DELTA.y.vertline..ltoreq..DELTA.V/V.sub.H,
.vertline..DELTA.z.vertline..ltoreq..DELTA.V/V.sub.H,
and the magnitude of .DELTA.x is such that any discrete portion
remains stable when subjected to a voltage V.sub.H
(1+.DELTA.x-.DELTA.y) applied for said second preselected length of
time.
15. The method of claim 14 wherein
.vertline..DELTA.x-.DELTA.y.vertline..ltoreq..DELTA.V/V.sub.H.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention pertains generally to liquid crystal cells and, more
particularly, to a matrix addressed liquid crystal optical
display.
2. Art Background
Liquid crystals are liquids whose molecules display ordering. This
ordering is characterized by a localized alignment of the liquid
crystal molecules. The direction of this localized alignment and
thus the spatial orientation of the liquid crystal molecules can be
changed by the application of electric fields to produce
corresponding changes in the optical properties of the liquid
crystal. For example, a change in the spatial orientation of a
liquid crystal produced by the application of an electric field
affects the polarization of light, e.g., visible light in the 4500
to 8000 Angstrom range, incident on the liquid crystal. This change
in the polarization of incident visible light can be perceived, for
example, by viewing the liquid crystal between a polarizer and
analyzer. That is, the change in the polarization of the incident
light will result in a change in the amount of the light which is
transmitted through the liquid crystal when the liquid crystal is
arranged between an appropriately oriented polarizer and
analyzer.
A device in which a liquid crystal is confined between two bounding
surfaces, at least one of which is transparent to light, is called
a liquid crystal cell. Typically, two glass plates are used to
confine the liquid crystal. In addition, a plurality of electrodes
is usually applied to the glass plates in order to subject discrete
portions of the liquid crystal, referred to herein as pixels, to
electric fields to thereby alter the spatial configurations, and
thus the optical transmission properties, of the pixels. Thus, if
some pixels are made to transmit incident light while others do
not, an optical effect is produced which can be used to display
information.
In one particular type of liquid crystal cell, called a liquid
crystal twist cell, the liquid crystal molecules exhibit at least
two different spatial configurations when subjected to appropriate
electric fields. In at least one of these configurations the
molecules assume a twisted, helical configuration, with the axis of
the helix oriented perpendicularly with respect to the bounding
surfaces. On passing through the liquid crystal twist cell, the
plane of polarization of plane polarized incident light is rotated
by the helical orientation of the molecules. When the liquid
crystal molecules are in the second configuration, the different
orientation of the molecules has a different effect on the
polarization of the incident light. By, for example, placing the
liquid crystal twist cell between crossed polarizers, pixels in one
or the other spatial configuration will either transmit incident
light through the cell or not transmit light, and thus appear to be
light or dark.
Liquid crystal cells have been used in relatively small optical
displays, having less than about 100 pixels. These relatively small
displays include the now common liquid crystal wristwatches. In
many of these relatively small displays the spatial orientation of
the liquid crystal of each pixel is regulated by voltage pulses
transmitted through an individual electrical lead connected to just
that pixel of the display, so that each pixel is individually
electrically driven. While the use of individually driven leads is
considered acceptable in relatively small optical displays, the
cost and complexity of using individually driven leads in a large
optical information display, consisting of more than about 100
pixels, is presently prohibitive.
Because of their low power consumption and thin profile, liquid
crystal cells have also been used as components for flat panel,
large information optical displays, consisting of more than about
100 pixels. The optical transmission states of the many pixels of
such large displays are not regulated with individually driven
leads. Rather, in order to reduce costs and complexity, the pixels
of such large arrays are usually formed as the intersections of an
array of row and column electrodes, which intersections define a
matrix of liquid crystal pixels. In each liquid crystal pixel the
optical transmission is switched from one state to another by
applying an appropriate voltage to the pixel through the
interaction of voltages applied to the row and column electrodes
intersecting at the pixel. Selective switching of individual pixels
from one optical transmission state to another in such a matrix
array, without appreciably affecting other pixels, is referred to
as dynamic matrix addressing.
Dynamically matrix addressed, liquid crystal displays which do not
exhibit bistability are limited in size, i.e., are limited in the
number of addressable rows or columns. The particular size limit is
dependent on the type of scheme used to matrix address the display
as well as the properties of the display. A liquid crystal cell
which exhibits bistability is one which displays memory with
respect to two different spatial orientations of the liquid crystal
(which correspond to two different optical transmission states of
the cell). That is, the liquid crystal of the cell can be switched
to a new spatial orientation by applying, for example, a relatively
high voltage across the cell, and the liquid crystal remains in the
new orientation even if the voltage is entirely removed or reduced
to some lower, nonzero value, i.e., a holding voltage. In addition,
in a bistable cell the liquid crystal of the cell is switched to
its former orientation by, for example, applying a relatively low
voltage (lower than the holding voltage) across the cell and the
cell remains in this state even after the relatively low voltage is
removed or the voltage is returned to the holding voltage value.
The pixels of liquid crystal displays which do not exhibit
bistability require continual refreshing by an appropriate voltage
signal to maintain an optical contrast. In the case of a
nonbistable, liquid crystal matrix display which includes a liquid
crystal cell responsive to the root-mean-square (rms) value of an
applied AC field (which is the case with liquid crystal twist
cells), the upper limit on the number of rows or columns of the
matrix display is inversely proportional to the square of the ratio
of the rms-voltages required to produce an acceptable optical
contrast. Thus, an increase in the number of rows or columns of the
display may require a corresponding decrease of this ratio (see A.
R. Kmetz, in Nonemissive Electrooptic Displays, edited by Kmetz and
von Willisen (Plenum, N.Y., 1976), pp. 270-273 ). Because there are
practical limitations on the decreases in the ratios of these
rms-voltages which are achievable with liquid crystal displays, it
then follows that the number of rows or columns of liquid crystal
matrix displays which do not exhibit bistability is limited.
However, in principle, no such limit exists for liquid crystal
matrix displays which do exhibit bistability of states.
Consequently, an important objective of those attempting to perfect
large information liquid crystal matrix displays has been to
fabricate a liquid crystal matrix display which exhibits
bistability.
Efforts directed at developing bistable liquid crystal displays
have resulted in the development of temporarily bistable liquid
crystal cells (see, e.g., E. P. Raynes, in Nonemissive Electrooptic
Displays, edited by Kmetz and von Willisen (Plenum, N.Y., 1976),
pp. 29-36). A temporarily bistable liquid crystal cell is one which
exhibits two states. The cell is switched to one of these states by
applying a voltage greater than, or equal to, a threshold voltage
across the cell. If the voltage is removed, the cell quickly
reverts to the other state, but this reversion can be retarded for
a time by applying a biasing voltage lower than the threshold
voltage.
Two matrix addressing schemes which have been used with temporarily
bistable liquid crystal matrix displays are the "three-to-one" and
"two-to-one" matrix addressing schemes. The application of these
matrix addressing schemes to temporarily bistable liquid crystal
matrix displays has been reviewed by Kmetz in Nonemissive
Electrooptic Displays, edited by Kmetz and von Willisen (Plenum,
N.Y., 1976), pp. 268-269. Both of these addressing schemes employ
biasing voltages to retard the relaxation of the liquid crystal
from one state to the other. Because these matrix displays are only
temporarily bistable and thus require periodic refreshing voltage
signals, these displays are limited in their multiplexing
capacities (the number of addressable columns or rows). The upper
limits on the multiplexing capacities of these displays differ
depending on which addressing scheme, e.g., three-to-one or
two-to-one, is employed. In general, it cannot be predicted which
addressing scheme is preferable for a particular device. For one
particular temporarily bistable device reviewed by Kmetz, supra, p.
269, some improvement in writing speed and multiplexing capacity is
obtained, at the cost of higher operating voltages and some
flicker, by the use of the two-to-one addressing scheme rather than
the three-to-one addressing scheme.
A liquid crystal twist cell which is truly bistable, rather than
merely temporarily bistable, has been disclosed in U.S. Pat. No.
4,239,345, issued on Dec. 16, 1980, to D. W. Berreman and W. R.
Heffner. The cell is characterized by at least two stable states,
either of which is stable as long as no external, threshold energy,
e.g., no voltage in excess of some threshold voltage, is applied to
the cell. External energy is necessary only for switching the cell
between the stable states. The potential inherent in this or any
other bistable, liquid crystal twist cell, i.e., of using a truly
bistable cell to fabricate a large information optical display
which, in principle, is not limited in size, has not yet been
realized.
SUMMARY OF THE INVENTION
The present invention is a dynamically matrix addressed, bistable
liquid crystal optical display. The optical display includes a
bistable liquid crystal twist cell of the type described in Proc.
SID 22, 191 (1981), and in U.S. patent application Ser. No.
198,294, U.S. Pat. No. 4,505,548, filed Oct. 20, 1980, by D. W.
Berreman and W. R. Heffner, which is hereby incorporated by
reference. A form of matrix addressing used in the liquid crystal
optical display not only allows the display to exploit the
advantages inherent in its bistability, but enhances the
operational characteristics of the display, when compared with
other matrix addressing schemes.
The liquid crystal twist cell included in the optical display of
the present invention exhibits two stable states, either of which
is maintained by application of a given holding voltage. Switching
between the two stable states is effected by application of a
switching voltage, which lies outside a holding voltage range over
which the cell is bistable, for the short period of time necessary
to convert the liquid crystal from one state to another. This
switching is effected without passing disclinations
(discontinuities in the orientations of the liquid crystal
molecules) across the cell. Subsequent to switching, the voltage is
returned to a voltage in the holding voltage range and the cell
remains stable in the new configuration.
In the optical display of the present invention all pixels of one
or more rows or one or more columns are nonselectively and
substantially simultaneously "cleared" (the pixels do not transmit
light when arranged between crossed polarizers or when other means
for producing an optical change is used) by applying, to all pixels
in the one or more rows or one or more columns, a voltage below the
holding voltage range for a sufficient length of time to switch the
pixels to one of the stable states of the liquid crystal. Selected
pixels are then "written" (the selected pixels transmit light when
arranged between crossed polarizers or when other means for
producing an optical change is used) by applying, to the selected
pixels, a voltage above the holding voltage range for a sufficient
length of time to switch the pixels to the other of the two stable
states of the liquid crystal. During the "write" procedure,
nonselected pixels are preferably subjected only to the holding
voltage. Subsequent to the "write" procedure all pixels are then
subjected to the holding voltage.
Among other advantages, the inventive optical display, when matrix
addressed with the procedure described above, exhibits a higher
operating speed than when matrix addressed with other schemes.
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the matrix addressed, liquid crystal optical display
of the present invention are described with reference to the
accompanying drawings, where:
FIG. 1 is an exploded, perspective view of one embodiment of the
bistable liquid crystal twist cell useful in the inventive optical
display;
FIG. 2 is a schematic representation of the liquid crystal
directors when the bistable liquid crystal twist cell useful in the
inventive optical display is in three different states denoted,
respectively, the DOWN state, the NO FIELD state, and the UP
state;
FIG. 3 is an energy diagram depicting the Gibbs free energy versus
the square of the applied voltage for the DOWN and UP states of the
bistable liquid crystal twist cell useful in the inventive optical
display;
FIG. 4 illustrates three-to-one and two-to-one matrix
addressing;
FIG. 5 is a graph of voltage versus thickness-to-pitch ratio
depicting one procedure for measuring the practical bistable
voltage range of the inventive optical display;
FIG. 6 is a graph of switching time versus thickness-to-pitch ratio
depicting one procedure for measuring T.sub.SW (on) and T.sub.SW
(off) for the inventive optical display;
FIG. 7 is a graph of the switching time versus the ratio of
switching voltage to holding voltage, for the CLEAR (U.fwdarw.D)
and WRITE (D.fwdarw.U) transitions of the bistable liquid crystal
twist cell useful in the inventive optical display;
FIG. 8 is a graph of the CLEAR (U.fwdarw.D) relative switching
times versus the thickness-to-pitch ratio of the bistable liquid
crystal twist cell useful in the inventive optical display at zero
switching voltage and at ambient temperatures of 40 degrees C.,
23.5 degrees C., and 13 degrees C.; and
FIG. 9 is a graph of the CLEAR (U.fwdarw.D) and WRITE (D.fwdarw.U)
relative switching times versus the thickness-to-pitch ratio of the
bistable liquid crystal twist cell useful in the inventive optical
display for different values of the ratio of switching voltage to
holding voltage.
DETAILED DESCRIPTION
The present invention is a matrix addressed, bistable, liquid
crystal optical information display which includes a plurality of
liquid crystal pixels conveniently arranged in a matrix of rows and
columns. Because the liquid crystal display is bistable, in
principle there is no limit on the size, i.e., on the number of
rows or columns, of the display. The content of the information
displayed by the liquid crystal display is changed by a form of
matrix addressing which enhances the operational characteristics of
the display. Thus, for example, this form of matrix addressing
enables the content of the information displayed by the present
invention to be changed more quickly, and more reliably, than if
other forms of addressing were used.
The matrix-like, liquid crystal optical display of the present
invention includes a bistable liquid crystal twist cell of the type
disclosed in U.S. patent application Ser. No. 198,294, U.S. Pat.
No. 4,505,548, filed Oct. 20, 1980, by D. W. Berreman and W. R.
Heffner, which cell, as used in the inventive display, includes a
relatively large number of pixels, typically in excess of about
100. Although the bistable liquid crystal twist cell is fully
described in U.S. patent application Ser. No. 198,294, for the sake
of completeness and continuity the cell is briefly described
below.
With reference to FIG. 1, one embodiment of the bistable liquid
crystal twist cell 10 useful in the optical display of the present
invention includes two bounding surfaces 16 and 22. These bounding
surfaces 16 and 22 serve to confine the liquid crystal material of
the cell 10 between them and define the upper and lower boundaries
of the liquid crystal material. Typically, the distance between
these surfaces, defined as the thickness of the liquid crystal
cell, ranges from about 1 micron to about 100 microns, and
preferably from about 5 microns to about 20 microns. At least one
of the bounding surfaces, surface 16, transmits electromagnetic
radiation of interest. Thus, for example, if the optical display of
the present invention is to display information to humans, then
surface 16 transmits incident visible light in the 4500 to 8000
Angstrom range. On the other hand, if the inventive optical display
is used to communicate with machines, such as computers, then
surface 16 transmits the incident electromagnetic radiation used
for this purpose, e.g., infrared radiation at wavelengths greater
than about 8000 Angstroms. Two convenient bounding surfaces 16 and
22 for transmitting visible light are the lower and upper surfaces,
respectively, of two glass plates 14 and 20.
The liquid crystal twist cell 10 also includes electrodes which
define the liquid crystal pixels included in the optical
information display of the present invention. In one embodiment the
electrodes consist of one or more electrically conductive strips 18
and 24 of, for example, indium oxide evaporated onto each of the
bounding surfaces 16 and 22. The electrodes 18 on the upper
bounding surface 16 are pictured in FIG. 1 as being parallel to one
another, although they need not be so provided they do not
electrically contact one another, and the electrodes 24 on the
lower bounding surface 22 are also pictured as being parallel to
one another, although they need not be so provided they do not
electrically contact one another. In addition, the electrodes 18
and 24 need not be straight, as pictured in FIG. 1, but can have
curved shapes, e.g., shapes characteristic of alphanumeric
characters. Moreover, the width of the electrodes 18 and 24 need
not be uniform. However, the electrodes 18 and 24 are arranged
transversely (not parallel) with respect to each other. By
projecting the electrodes 18 down onto the electrodes 24, discrete
portions or columns 19 of liquid crystal are defined, which columns
constitute the pixels of the optical display of the present
invention. Thus, for example, if the electrodes 18 and 24 are
straight, then each pair of transversely oriented electrodes
defines one pixel. If a liquid crystal twist cell 10 has n straight
electrodes oriented transversely with respect to m straight
electrodes, it thus has n.times.m pixels.
In principle, there is no upper limit on the width of the indium
oxide strips 18 and 24. However, the width of these strips is
preferably no smaller than about 0.001 inches. At widths smaller
than about 0.001 inches, the lateral dimensions of the pixels
(defined by the projections of indium oxide strips onto
transversely oriented oxide strips) become comparable to their
height (the distance between the bounding surfaces 16 and 22). This
results in a degradation of the preferred manner of operation of
the liquid crystal twist cell, described below.
The liquid crystal twist cell 10 useful in the optical display of
the present invention is distinguished from most other liquid
crystal cells by having two stable states, either of which may be
maintained by the same holding voltage. These stable states have
different spatial molecular orientations. In addition, the liquid
crystal twist cell is switched from one stable state to the other
without passing a disclination (a line of discontinuity in the
orientation of the liquid crystal molecules) through the cell.
The optical transmission properties of the two stable states of the
liquid crystal twist cell 10 differ. That is, each state affects
the polarization of incident, plane polarized light differently.
The liquid crystal twist cell 10 includes apparatus for optically
distinguishing between the two states. This apparatus takes the
convenient form of a polarizer 12, a crossed analyzer 26, and a
reflector 28, as shown in FIG. 1.
The bistability of the liquid crystal twist cell 10 is between two
more or less helically twisted states. This bistability is achieved
by satisfying two conditions. The first condition which should be
satisfied is that the liquid crystal have an unstrained pitch
approxmately equal to the thickness of the liquid crystal layer.
The term "pitch" denotes the distance in which the liquid crystal
directors rotate through a full helical turn. The liquid crystal
directors are oriented lines used to indicate the orientation of
the liquid crystal molecules (see, e.g., P. G. DeGennes, Physics of
Liquid Crystals (Clarendon Press, Oxford 1975), p. 7, for a more
complete explanation of the concept of liquid crystal directors).
The term "unstrained" in the phrase "unstrained pitch" denotes the
fact that the liquid crystal assumes a helically twisted
configuration without the application of a torque. Hereinafter, the
term "pitch" is used to denote "unstrained pitch." Thus, to satisfy
the first condition, the ratio of the thickness of the liquid
crystal layer, t, to the pitch, P, of the liquid crystal should be
approximately equal to one. The range of the thickness-to-pitch
ratio, t/P, over which the liquid crystal twist cell remains
bistable is dependent on the material properties of the liquid
crystal, the total rotation of the liquid crystal directors, as
well as the inclination of the liquid crystal directors at the
bounding surfaces, as discussed below. By way of example, and for
typical liquid crystal materials, the cell is bistable if t/P is in
the range from about 0.8 to about 1.2. A thickness-to-pitch ratio
of about one is achieved by, for example, adding an appropriate
amount of chiral dopant (a dopant which induces helical twist in
the liquid crystal), such as cholesteryl nonanoate, CN, to a
nematic liquid crystal such as the cyanobiphenyl-terphenyl mixture
sold under the trade name E-7 and which can be purchased from the
British Drug House. It has been found that the relation between
pitch, P, in microns, and concentration of CN,C, in E-7, in percent
by weight of CN, is given by the formula
which enables one to determine the appropriate concentration of CN
in E-7 necessary to achieve a desired pitch.
Liquid crystal materials are characterized by a number of
parameters, including the elastic constants K.sub.1, K.sub.2 and
K.sub.3 (see, e.g., P. G. DeGennes, supra, p. 63). The liquid
crystal materials useful in the liquid crystal twist cell 10 are
those where K.sub.1 is approximately equal to K.sub.3 and K.sub.2
is approximately one-half K.sub.1 or K.sub.3. Thus, other nematic
liquid crystals, besides E-7, whose elastic constants satisfy this
relationship and which are useful with the liquid crystal twist
cell 10 include the nematic liquid crystal sold under the trade
name ROTN 619, and which can be purchased from Hoffmann-La Roche,
Inc. of Nutley, N.J. Chiral dopants which are added to ROTN 619 to
produce a t/P of about one include the chiral dopant sold under the
trade name CB-15, and which can be purchased from EM Laboratories,
Inc. of Elmsford, N.Y.
The second condition which should be satisfied to achieve
bistability is that the liquid crystal directors at one or both of
the bounding surfaces 16 and 22 be obliquely inclined relative to
the bounding surfaces. Preferably, the inclination of the liquid
crystal directors at one or both of the bounding surfaces 16 and
22, relative to these surfaces, is in the range of about 20 degrees
to about 50 degrees. This inclination is achieved, for example, by
evaporating silicon-monoxide onto the bounding surfaces 16 and 22
at an angle of about 5 degrees relative to these surfaces. The
silicon monoxide induces an oblique inclination of the liquid
crystal directors at the bounding surfaces. Satisfying the two
conditions described above, with the liquid crystal directors
obliquely inclined at both bounding surfaces, produces a torque on
the liquid crystal, the axis of which torque is generally aligned
with the helical twist axis of the liquid crystal and thus
perpendicular to the bounding surfaces. This is in contrast to
other liquid crystal twist cells, such as the liquid crystal twist
cell described by W. Greubel in Appl. Phys. Lett. 25, 5 (1974 ),
where the liquid crystal directors at the bounding surfaces are not
obliquely inclined to, but rather perpendicular to, the bounding
surfaces and the liquid crystal is not subject to a torque.
As noted above, one of the two conditions which should be satisfied
for bistability is that t/P be about one. For E-7 doped with CN,
for an inclination of the liquid crystal directors at the bounding
surfaces of about 33 degrees, and for a rotation of the liquid
crystal directors about the helical twist axis of about 360
degrees, t/P ranges from about 0.8 to about 1.2 (the range of t/P
is dependent on the inclination of the directors at the bonding
surfaces). Values of t/P greater than about 1.2 are undesirable
because the liquid crystal twist cell ceases to be bistable. Values
of t/P less than about 0.8 are undesirable because the time
required to switch from one of the two stable states to the other
becomes undesirably long. For a particular liquid crystal material,
an appropriate t/P is easily determined by employing a control
sample.
The two stable, helically twisted states of the liquid crystal
twist cell 10, referred to here as the DOWN state and the UP state,
are shown in FIG. 2. In the DOWN state, the liquid crystal
directors rotate approximately a full turn through the liquid
crystal layer while the inclination angle of the directors relative
to the bounding surfaces 16 and 22 remains constant. The amount of
the rotation ranges from about 265 degrees to about 400 degrees,
and preferably from about 355 to about 365 degrees. The DOWN state
is similar to the configuration when no field is applied. The UP
state differs from the DOWN state in that in the middle of the
liquid crystal layer, the liquid crystal directors are nearly
perpendicular to the bounding surfaces 16 and 22.
When a pixel 19 of the liquid crystal twist cell 10 is in the DOWN
state, the plane of polarization of incident light is rotated
through approximately a full turn, i.e., through about 360 degrees,
and consequently the pixel appears dark or "off" between the
crossed polarizer 12 and analyzer 26. When the pixel 19 is in the
UP state, the incident light becomes elliptically polarized during
its passage through the pixel, and a portion of this elliptically
polarized light exits the pixel polarized perpendicularly with
respect to the incident direction and therefore parallel to the
analyzer 26. This component, parallel to the analyzer 26, contains
as much, or even more, than half of the light energy transmitted
through the liquid crystal cell. Therefore, a pixel 19 in the UP
state transmits at least a portion of the incident light through
the cell and thus appears to be "on".
In other embodiments of the liquid crystal twist cell 10 the DOWN
state rather than the UP transmits light through the cell. In these
embodiments the DOWN state transmits light because, for example, a
quarter wave plate is placed between the lower glass plate 20 and
the analyzer 26. The quarter wave plate converts elliptically
polarized light, such as that produced by the UP state, to plane
polarized light. By appropriately tuning the quarter wave plate and
appropriately orienting the analyzer 26 relative to the quarter
wave plate, the DOWN state transmits incident light and the UP
state does not.
In one embodiment of the liquid crystal twist cell 10, the liquid
crystal is responsive to AC electric fields of frequency about 20
to about 200,000 Hz and preferably of frequency about 40 to about
600 Hz. In fact, the spatial state of the liquid crystal is a
function of the rms-value of the applied AC voltage at any chosen
frequency in the above range. However, the liquid crystal is
insensitive to voltage polarities at these frequencies.
The operation of the liquid crystal twist cell 10 is conveniently
described with reference to FIG. 3, which is a graph depicting,
qualitatively, the Gibbs free energy of the liquid crystal as a
function of the square of the rms-voltage (called the mean square
voltage) applied across the cell. For the sake of simplicity, the
term voltage is used in this disclosure to denote an rms-voltage.
In FIG. 3, the line joining points a and c represents the energy
curve for the DOWN state, while the line joining points e and g
represents the energy curve for the UP state. The curve ec
represents an intermediate barrier state between the UP and DOWN
states. Moreover, the voltage range between the voltages associated
with the points e and c represents a range of holding voltages over
which the liquid crystal twist cell 10 is bistable.
For voltages less than the voltage associated with the point e,
only the DOWN state exists. As the voltage is increased to some
value between the voltages associated with the points e and c,
i.e., to some value in the holding voltage range such as V.sub.h,
the cell still remains in the DOWN state. However, once the voltage
is increased beyond that associated with the point c, i.e., once
the voltage exceeds the holding voltage range, the cell switches to
the UP state. After the cell has switched to the UP state, the
voltage may be returned to any value in the bistable range, e.g.,
to V.sub.h, and the cell will remain in the UP state. Lowering the
voltage below that associated with the point e results in the cell
switching from the UP state to the DOWN state. Returning the
voltage to the bistable region, e.g., to V.sub.h, leaves the cell
in the DOWN state. Operation of the cell thus involves application
of a holding voltage chosen from the holding voltage range to a
pixel 19 of the liquid crystal twist cell 10, and the application
of a voltage outside the holding voltage range to the pixel only
when switching is desired. That is, switching is accomplished by
raising or lowering the voltage to values outside the holding
voltage range for a sufficient duration of time. The minimum
duration of time needed to achieve switching is called the
switching time.
In the above description of the operation of the liquid crystal
twist cell 10, it was assumed that the point e lies to the right of
the vertical axis, i.e., the energy axis, in FIG. 3. But in some
particular cells point e does, in fact, lie to the left of the
vertical energy axis. In this situation, switching from the UP to
the DOWN state is not accomplished just by decreasing the voltage
to a value below that associated with the point e. However,
switching from the UP to the DOWN state is accomplished by
employing fluid dynamic effects (see columns 4-5 of U.S. Pat. No.
4,239,345, which is hereby incorporated by reference), or
dielectric anisotropy reversal with AC fields of frequency greater
than about 10 kHz (see Gerritsma et al, Solid State Comm. 17, 1077
(1975)).
In principle, the liquid crystal twist cell 10 useful in the
present invention is indefinitely bistable. That is, in principle,
there exits a holding voltage range over which either the UP state
or the DOWN is stable indefinitely. Obviously, this is not
verifiable in practice. However, a holding voltage is capable of
maintaining either state for at least 5 minutes, and in fact
significantly longer than 5 minutes.
The liquid crystal optical display of the present invention
includes a version of the liquid crystal twist cell 10, described
above, with a relatively large number of transversely oriented
electrodes 18 and 24, e.g., the transversely oriented indium oxide
strips, on bounding surfaces 16 and 22 defining a relatively large
matrix of pixels. For example, the liquid crystal optical display
includes 4 or more rows of indium oxide strips intersecting 4 or
more columns of indium oxide strips, thereby defining an array of
pixels, arranged in rows and columns, numbering 16 or more.
Preferably, the liquid crystal optical display includes 10 or more
rows of indium oxide strips intersecting 10 or more columns of
indium oxide strips, defining 100 or more pixels. By applying
appropriate voltage pulses in an appropriate sequence to the row
and column indium oxide electrodes, that is, by dynamically matrix
addressing the optical display, information is displayed.
The dynamic matrix addressing procedure used in the present
invention is such that the information displayed by the inventive
optical display is changed in two steps. First, a voltage pulse
whose amplitude is smaller than the holding voltage, e.g., zero
voltage, and of duration equal to the switching time, is
nonselectively applied to all pixels of a row or of all rows (or of
a column or of all columns) through the row and column electrodes,
in order to cause these pixels to switch from the UP state (if they
are in this state) to the DOWN state. That is, all the pixels of
one or more rows (or one or more columns) are nonselectively,
substantially simultaneously switched from the UP state to the DOWN
state. The term "substantially simultaneously" is used to denote
the fact that the voltage pulse used to switch the pixels from the
UP state to the DOWN state may not be received by all the pixels of
the one or more rows (or one or more columns) at exactly the same
instant of time. Thus the pixels may not switch at exactly the same
instant. However, it is only necessary that this voltage pulse
reach all the pixels over a relatively short period of time, e.g.,
a time comparable to the period of the applied AC voltage, for the
inventive display to operate in a desirable fashion. In the DOWN
state the pixels transmit no light and thus appear dark or "off."
The switching of a pixel from the UP state to the DOWN state is
referred to in this disclosure as a CLEAR (U.fwdarw.D) transition.
The second step involves the selective application of a voltage
pulse of amplitude greater than the holding voltage and of duration
equal to or greater than the switching time to particular pixels,
through the row and column electrodes, in order to cause these
particular pixels to switch from the DOWN state (if they are in
this state) to the UP state. In the UP state the pixels transmit
light and thus appear to be "on." The switching of a pixel from the
DOWN state to the UP state is referred to this disclosure as a
WRITE (D.fwdarw.U) transition.
By way of example, FIG. 4(a) illustrates the matrix addressing
procedure used in a preferred embodiment of the present invention.
This preferred embodiment employs three-to-one addressing, an
addressing scheme wherein all pixels of one or more rows or one or
more columns are nonselectively, substantially simultaneously
CLEARED, and then pixels are selectively WRITTEN. Three-to-one
matrix addressing is conveniently described with reference to the
terms "selected pixel," "selected row," and "selected column." A
"selected pixel" is one which has been selected to undergo a WRITE
transition, and is thus a pixel which has been selected to transmit
light (when viwed between the crossed polarizer 12 and analyzer
26). The circled pixel in FIG. 4(a) is "selected pixel." "Selected
rows" and "selected columns" are those containing "selected
pixels." Thus, a "selected pixel" is one at the intersection of a
"selected row" and "selected column."
In the preferred embodiment, as in all embodiments of the present
invention, all the pixels of one or more rows (and/or of one or
more columns) of the optical display are initially nonselectively,
substantially simultaneously CLEARED. This is accomplished, for
example, by applying a voltage pulse, of duration equal to or
greater than the switching time, to the row electrode containing
the row of pixels to be cleared, and an identical voltage pulse to
all the column electrodes. A pixel is, by definition, just the
column of liquid crystal defined by the projection of one electrode
on the bounding surface 16 onto a transversely oriented electrode
on the bounding surface 22. Thus, when voltages are applied to the
transversely oriented electrodes which define a pixel, the pixel
"sees" only the difference in the voltages applied to the
electrodes. Consequently, each pixel in the row of pixels to be
CLEARED experiences a zero voltage pulse (zero voltage being below
the holding voltage range and therefore leading to a CLEAR
transition) because each pixel in the row "sees" a voltage equal to
the difference between the identical column and row voltages.
After one or more rows (or one or more columns) of pixels of the
preferred embodiment of the optical display are nonselectively,
substantially simultaneously CLEARED, the preferred embodiment is
then WRITTEN. That is, selected pixels are caused to undergo a
WRITE transition. A selected pixel is caused to undergo a WRITE
transition by applying to the selected column electrode a voltage
pulse of amplitude 2 V.sub.H and of duration equal to or greater
than the switching time, while the other nonselected column
electrodes are maintained at zero voltage. Here, V.sub.H denotes
the holding voltage applied to the pixels of the preferred
embodiment, which voltage may take on any value in the holding
voltage range (see FIG. 3). For purposes of illustration, the
voltage V.sub.H is assumed to be equal to the voltage midway
between the voltages at the ends of the holding voltage range.
Simultaneously with the application of the 2 V.sub.H and zero
voltages to the selected and nonselected column electrodes, the
selected row electrodes receive a voltage pulse of amplitude
-V.sub.H of duration equal to the switching time while the
nonselected row electrodes are held at V.sub.H. The selected
pixels, i.e., the pixels at the intersections of the selected
column and row electrodes, thus experience a voltage pulse of
amplitude 3 V.sub.H (the difference between the selected column and
row voltages) while the nonselected elements are all maintained at
the holding voltage (that is, at +V.sub.H or -V.sub.H, the pixels
being unaffected by the voltage polarities). Of course, the
voltages applied to the row electrodes and column electrodes are
interchangeable.
There are two variations of the preferred embodiment of the
inventive optical display which employ different WRITE procedures.
In the first variation row-by-row scanning is employed. That is,
each row electrode is sequentially subjected to a voltage pulse of
amplitude -V.sub.H while all other row electrodes are subjected to
a voltage pulse of amplitude +V.sub.H. Simultaneously, as a scanned
row electrode is subjected to the voltage pulse of amplitude
-V.sub.H, column electrodes containing selected pixels in the row
are subjected to a voltage pulse of amplitude 2 V.sub.H while all
other column electrodes are subjected to zero voltage. In the
second variation, column-by-column scanning is employed. That is,
each column electrode is sequentially subjected to a voltage pulse
of amplitude 2 V.sub.H while all other column electrodes are
subjected to a zero voltage pulse. Simultaneously, row electrodes
containing selected pixels which fall within the scanned column
electrode are subjected to a voltage pulse of amplitude -V.sub.H
while all other row electrodes are subjected to a voltage
+V.sub.H.
In order to matrix address the inventive optical display, as
described above, certain operational characteristics of the optical
display should be determined. Among these are the bistable holding
voltage range of the display, as well as the voltage pulse
durations necessary to produce CLEAR (U.fwdarw.D) and WRITE
(D.fwdarw.U) transitions, once the voltage pulse amplitudes have
been chosen. Thus, for example, if CLEAR transitions are to be
produced with a zero voltage pulse, the switching time at zero
volts should be known. Similarly, if WRITE transitions are to be
produced with a voltage pulse of amplitude, for example, three
times the holding voltage, the switching time at three times the
holding voltage should be known.
Variations in the thickness-to-pitch ratio, t/P, of the liquid
crystal twist cell included in the inventive optical display (small
thickness variations are almost unavoidable) significantly affect
both the extent of the holding voltage range and the switching
times of the inventive optical display. An acceptable extent of a
holding voltage range is one which is one half percent or more of
the mean value of the range. If the extent of the holding voltage
range is smaller than this, the holding voltage range is said to be
pinched off. Thus, for example, if the liquid crystal is E-7 doped
with CN, then as t/P is increased above 1.0 (t/P should be
approximately equal to 1.0 for bistability), it is known that the
holding voltage range narrows and is pinched off at t/P
approximately equal to 1.15. Thus, for E-7 doped with CN, t/P is
limited to values below 1.15, and preferably below about 1.10. In
addition, it is known that switching times for CLEAR transitions
increase sharply for values of t/P below about 0.95. Thus, if the
liquid crystal is E-7 doped with CN, the variation of the
thickness-to-pitch ratio of the liquid crystal twist cell included
in the present invention is preferably from about 0.95 to about
1.10 . Within this thickness-to-pitch range, switching times for
the inventive optical display for both WRITE and CLEAR transitions
are chosen, as described below, to be of sufficient duration to
ensure that all pixels undergo switching.
A "practical" holding voltage range is one over which the inventive
optical display is bistable for at least some minimum length of
time, e.g., 5 minutes. One procedure for determining the
"practical" bistable voltage range and the switching times for the
optical display of the present invention is to determine these
parameters for a control sample, i.e., a sample liquid crystal
twist cell useful in the optical display. The control sample, as
well as the inventive optical display, will almost always exhibit
some variation in t/P as well as other imperfections. Thus, the
practical bistable voltage range of the inventive optical display
is determined by measuring and plotting, for the control sample,
both the maximum voltage at which the DOWN state is stable and the
minimum voltage at which the UP state is stable as a function of
the t/P of the control sample, as shown in FIG. 5. If the
thickness-to-pitch ratio of the control sample varies, for example,
for 0.95 to 1.05, then the upper bound on the practical bistable
voltage range is just the voltage intercept associated with the
intersection of the line t/P=0.95 with the curve describing the
maximum voltage at which the DOWN state is stable (the upper curve
in FIG. 5). The lower bound on the useful bistable voltage range is
just the voltage intercept associated with the intersection of the
line t/P=1.05 with the curve describing the minimum voltage at
which the UP state is stable (the lower curve in FIG. 5). Having
determined the upper and lower bounds of the practical bistable
voltage range, this voltage range is conveniently defined in terms
of some mean value, V.sub.H, and some increment, .DELTA.V, as shown
in FIG. 5. Thus, the practical bistable voltage range is specifed
as V.sub.H .+-..DELTA.V.
Having determined the practical bistable voltage range, the optical
display is matrix addressed once appropriate switching times for
"on" and "off" switching, i.e., WRITE and CLEAR transitions, are
known. For example, if a zero voltage pulse is to be used to
produce a CLEAR transition and a voltage pulse of amplitude three
times the holding voltage is to be used to produce a WRITE
transition, then switching times at zero volts and at three times
the holding voltage (as determined above) should be determined.
This is done by measuring, and plotting, for the control sample,
the switching time, T.sub.S, at zero volts and at three times the
holding voltage as a function of the measured t/P of the control
sample, as shown in FIG. 6. From this curve a switching time is
chosen, T.sub.SW (on), sufficient to turn "on" the whole of the
control sample when it is subjected to three times the holding
voltage (as shown in FIG. 6). Similarly, another switching time is
chosen, T.sub.SW (off), sufficient to turn "off" the whole of the
control sample when it is subjected to zero volts (as shown in FIG.
6). These values of T.sub.SW (on) and T.sub.SW (off) are applicable
to the optical display. Of course, switching times greater than
T.sub.SW (on) and T.sub.SW (off) are also useful.
Once the practical bistable voltage range, as well as T.sub.SW (on)
and T.sub.SW (off), of the optical display have been determined, as
described above, then a two-step matrix addressing scheme wherein
the pixels of a row or of all rows (and/or of a column or all
columns) are first nonselectively, simultaneously CLEARED and then
selectively WRITTEN is readily applied to the inventive optical
display.
It has been found, empirically, but readily, that for the preferred
embodiment of the present invention the voltages applied to the
selected columns, selected rows, and nonselected rows need not be
exactly equal to, respectively, 2 V.sub.H, -V.sub.H, and +V.sub.H.
Here V.sub.H denotes the mean value of the practical bistable
voltage range of the optical display. Rather, there is a
permissible variation in these voltages related to the increment
.DELTA.V used in specifying the practical bistable voltage range.
If the selected column voltage is defined to be
(2+.DELTA.x)V.sub.H, the nonselected row voltage is defined to be
(1+.DELTA.y)V.sub.H, and the selected row voltage is defined to be
-(1+.DELTA.z)V.sub.H, where .DELTA.x, .DELTA.y, and .DELTA.z are
positive or negative increments, then these increments take on the
following useful values if row-by-row scanning is employed:
(1) .vertline..DELTA.y.vertline..ltoreq..DELTA.V/V.sub.H ;
(2) .vertline..DELTA.x-.DELTA.y.vertline..ltoreq..DELTA.V/V.sub.H ;
and
(3) V.sub.H (1+.DELTA.z) should be such that any pixel of the
inventive optical display remains stable during T.sub.SW (on). This
conditions is satisfied if .vertline..DELTA.z.DELTA. is taken to
be, for example,
.vertline..DELTA.z.vertline..ltoreq..DELTA.V/V.sub.H.
If column-by-column scanning is employed, then the increments
.DELTA.x, .DELTA.y, and .DELTA.z take on the following useful
values:
(1) .vertline..DELTA.y.vertline..ltoreq..DELTA.V/V.sub.H ;
(2) .vertline..DELTA.z.vertline..ltoreq..DELTA.V/V.sub.H ; and
(3) V.sub.H (1+.DELTA.x-.DELTA.y) should be such that any pixel of
the inventive optical display remains stable during T.sub.SW (on).
This condition is satisfied if
.vertline..DELTA.x-.DELTA.y.vertline. is taken to be, for example,
.vertline..DELTA.x-.DELTA.y.vertline..ltoreq..DELTA.V/V.sub.H.
EXAMPLE 1
One embodiment of the matrix addressed, bistable, liquid crystal
optical display of the present invention was fabricated as
described below. This display was fabricated to include an array of
16 pixels defined by the intersections of 4 row electrodes and 4
column electrodes.
The bistable liquid crystal twist cell included in the preferred
embodiment of the inventive optical display was fabricated using
two indium oxide-coated, rectangular glass plates purchased from
the Practical Products Company of Cincinnati, Ohio. The indium
oxide coating on each glass plate was transparent, and each glass
plate had dimensions of 1".times.11/2".times.5/8". Selected
portions of the indium oxide coating on each glass plate were
photolithographically removed, leaving four parallel strips of
indium oxide on each glass plate, each strip having a width of
about 1 mm. The surface of each glass plate bearing the indium
oxide strips was then overcoated with a layer of silicon monoxide
about 100 Angstroms thick by evaporating the silicon monoxide onto
each glass plate at a grazing angle of about 5 degrees relative to
the surface of each glass plate.
The two glass plates were oriented with respect to one another and
separated with two 13 .mu.m-thick mylar spacers placed along
opposite edges of the glass plates. In orienting the glass plates,
the surfaces bearing the indium oxide strips faced each other and
the strips on one surface were perpendicular to the strips on the
other surface. In addition, the glass plates were oriented so that
the directions from which the silicon monoxide had been evaporated
onto the surfaces of the glass plates differed by 180 degrees.
Furthermore, one glass plate was longitudinally offset from the
other by about 1/4 inch.
The two glass plates were epoxied together along the two edges
separated by the mylar spacers, leaving two open edges (the offset
edges) for liquid crystal filling and electrode connections. Each
of the glass plates included an overchanging portion extending from
the liquid crystal twist cell (see FIG. 1). The sandwich of glass
plates was then cured at a temperature of about 100.degree. C. for
about one hour to stabilize the cell thickness. Thereafter, the
thickness of the cell was measured by counting interference fringes
of monochromatic light using a variable wavelength monochrometer as
the light source. Cell thickness variation was recorded by
photographing the interference fringes under sodium illumination.
The cell thickness and thickness variation were measured to be 16.2
.mu.m.+-.1.5%.
By applying a drop of a liquid crystal mixture to one of the open
sides of the cell, the space between the glass plates was filled
with the liquid crystal mixture by capillary action. The liquid
crystal mixture used included the cyanobiphenyl-terphenyl mixture
sold under the trade name E-7, which was purchased from EM
Laboratories, Inc. of Elmsford, N.Y. The E-7 was doped with 1.303
percent by weight of cholesteryl nonanoate to produce a
thickness-to-pitch ratio (based upon the above measured cell
thickness and cell thickness variation) of 1.126.+-.1.5%.
The inclination of the liquid crystal directors at the silicon
monoxide coated surfaces for undoped E-7 was measured by conoscopy
(see Crossland et al, J. Phys. D: Appl. Phys. 9, 2001 (1976)) in a
separate cell with no helical twist. An inclination of about 33
degrees relative to the coated surfaces was measured. It was
assumed that the inclination of the liquid crystal directors in the
cell with twist, the cell used in the optical display, was also 33
degrees.
Electrical contacts were made between the cell and external wires
through two Zebra strips (conductive, elastomeric strips). That is,
two Zebra strips were applied to the overhanging surfaces of the
glass plates bearing the indium oxide electrodes and extending from
the cell. Each Zebra strip was applied to one of the glass surfaces
bearing the indium oxide electrodes, and arranged transversely with
respect to the electrodes. Four external wires were mounted on each
Zebra strip, each of the wires communicating with just one of the
indium oxide strips through the Zebra strip (by virtue of the
unidirectional conductive properties of the Zebra strip).
Electrical signals are communicated to each of the indium oxide
electrodes through the external wires.
In order to determine the practical holding voltage range and the
switching times of the liquid crystal twist cell, first the
amplitude and then the duration of a 500 Hz voltage signal applied
to the pixels of the cell was varied while the optical transmission
states of all 16 pixels, as viewed between crossed polarizers, were
observed through a microscope. It was found that the practical
holding voltage range for the liquid crystal twist cell of the
optical display, at a room temperature of about 26.5.degree. C.,
was 1.67-1.69 volts. The term "practical" means that, over the
voltage range of 1.67-1.69 volts, the cell exhibited bistability
for at least 5 minutes. The holding voltage used in matrix
addressing the optical display was 1.68 volts. It was also found
that the switching times for the cell, i.e., T.sub.SW (on) and
T.sub.SW (off), were, respectively, about 18 msec and about 130
msec. Here, T.sub.SW (on) denotes the minimum pulse duration
required to produce a WRITE transition at all 16 pixels at a
voltage equal to three times the holding voltage (of 1.68 volts),
while T.sub.SW (off) denotes the minimum pulse duration required to
produce a CLEAR transition at all 16 pixels at zero volts.
All the pixels of the preferred embodiment were first CLEARED and
then a pattern of selected pixels was WRITTEN. If the notation
(i,j) is used to denote the row and column locations of a selected
pixel, then the selected pixels were at (1,2), (1,3), (2,1), (2,4),
(3,1), (3,4), (4,2), and (4,4). The time required to WRITE the
pattern of selected pixels, using row-by-row scanning, was 72 msec
(=T.sub.SW (on).times.4 rows=18.times.4).
EXAMPLE 2
The content of the information displayed by the optical display of
the present invention is changed by a two-step matrix addressing
procedure wherein all pixels of one or more or even all rows
(and/or of one or more or even all columns) are first
nonselectively, simultaneously CLEARED and then particular pixels
are selectively WRITTEN. This addressing procedure enables the
content of the information displayed by the present invention to be
changed much more quickly than if the pixels were first selectively
CLEARED and then selectively WRITTEN. This can be deduced from FIG.
7 which depicts the typical dependence of the switching time on the
switching voltage, V.sub.S, for both CLEAR transitions and WRITE
transitions, of the bistable liquid crystal twist cell useful in
the present invention, at room temperature (23.5.degree. C.).
The curves of FIG. 7 represent the switching characteristics of a
relatively uniform portion of a liquid crystal twist cell which was
fabricated in accordance with the procedure described in Example 1.
The liquid crystal was E-7 doped with CN. The relatively uniform
portion of the cell included three adjacent pixels which had a
thickness of about 15 microns with a thickness variation of
.+-.0.2%, and a t/P of about 0.983 and a corresponding t/P
variation of .+-.0.2%. The practical holding voltage range for the
three adjacent pixels, i.e., the voltage range over which the three
adjacent pixels were bistable for at least 5 minutes, was measured
to be 1.780 to 1.810 volts, and the holding voltage used in
measuring the switching characteristics of the three adjacent
pixels was 1.785 volts.
The switching characteristics of the three adjacent pixels were
measured by varying the rms-amplitude of a 500 Hz voltage signal
applied to the three pixels and varying the duration until
switching was accomplished. Simultaneously, with the cell arranged
between crossed polarizers, the optical transmission states of the
three adjacent pixels of the cell were observed through a
microscope. If all three pixels switched from one state to another,
complete switching was considered to have occurred. If only one or
two pixels switched, partial switching was considered to have
occurred. If no pixels switched, no switching was considered to
have occurred.
In FIG. 7 the switching times have been plotted on a logarithmic
scale against a normalized switching voltage V.sub.S /V.sub.H,
where V.sub.H denotes the holding voltage. It should be noted that
two parallel curves are given in each case. The top curve
represents the minimum time required to switch from one state to
the other. The lower curve represents the pulse duration below
which no switching occurs. The area between the curves represents
partial or incomplete switching. This is also the case for the
curves shown in FIGS. 8 and 9.
In FIG. 7, for V.sub.S /V.sub.H greater than 1.0, WRITE transitions
occur. In this region the switching time shows a rapid decrease as
the switching voltage is increased. The switching time at V.sub.S
=2 V.sub.H is about 52 msec and at V.sub.S =3 V.sub.H is about 20
msec. For V.sub.S /V.sub.H less than 1.0, CLEAR transitions occur.
These are considerably slower than the WRITE transitions. The
switching time at zero volts, the shortest switching time, is about
200 msec.
The matrix addressing scheme of the present invention, which
nonselectively CLEARS one or more rows (or one or more columns) of
pixels substantially simultaneously and then selectively WRITES
pixels one row or column at a time, is to be distinguished from
those addressing schemes which selectively CLEAR selected pixels
within a row one row at a time and then selectively WRITE pixels.
The application of the former addressing scheme, rather than the
latter, to the optical display of the present invention, results in
relatively fast operation of the inventive optical display. Because
the switching time for CLEAR transitions with the inventive optical
display is relatively slow, it follows that the former addressing
scheme, the one used with the present invention, permits the
information content of the display to be changed much more quickly
than if the latter addressing scheme were used.
EXAMPLE 3
Because the addressing scheme applied to the inventive optical
display nonselectively CLEARS one or more rows (or one or more
columns) of pixels substantially simultaneously rather than
selectively CLEARING selected pixels one row at a time, the
inventive optical display operates relatively quickly not only at
room temperature, but also at relatively low temperatures. This can
be inferred from FIG. 8.
FIG. 8 depicts the typical dependence of switching time as a
function of t/P for CLEAR transitions (at zero volts) of the liquid
crystal twist cell useful in the present invention. Because
switching times scale as the inverse of the square of the cell
thickness (1/t.sup.2), this figure depicts the dependence of
relative switching time, T.sub.S /t.sup.2, on t/P. In particular,
this figure depicts the influence of temperature on CLEAR switching
times.
The data plotted in FIG. 8 represent the switching characteristics
of the three adjacent pixels of the liquid crystal twist cell
described in Example 2. The switching characteristics of the three
pixels were measured at different concentrations of CN in E-7, to
produce different t/P ratios, and for temperatures of 40.0.degree.
C., 23.5.degree. C., and 13.0.degree. C., using the procedure
described in Example 2. The measurements at the three different
temperatures were made with the cell in a thermostated
enclosure.
As is evident from FIG. 8, switching times for CLEAR transitions
exhibit a sharp increase as the temperature decreases from
40.0.degree. C. to 23.5.degree. C. to 13.0.degree. C. These
switching times increase by a factor of about 2.5 for each
10.degree. C. decrease in temperature. Consequently, any addressing
scheme applied to the present invention which CLEARS selected
pixels within a row one row at a time will result in relatively
slow operation at low temperatures. Because the addressing scheme
applied to the present invention nonselectively CLEARS one or more
rows of pixels simultaneously, it follows that the present
invention is able to operate at relatively high speed even at low
temperatures.
For purposes of contrast two-to-one addressing, an addressing
scheme which selectively CLEARS pixels and then selectively WRITES
pixels, is described below. The use of this addressing scheme in
the context of the inventive optical display not only results in
relatively slow operation of the display, but also produces adverse
effects not associated with other displays.
Two-to-one addressing is depicted in FIG. 4(b). In two-to-one
addressing, one or more selected column electrodes are subjected to
a voltage pulse of amplitude V.sub.H +.delta.V and of duration
equal to the switching time while all other column electrodes are
maintained at the holding voltage, V.sub.H. Simultaneously, the
selected row electrode is subjected to a voltage pulse of amplitude
-.delta.V of duration equal to the switching time while all other
row electrodes are maintained at zero voltage. Consequently, the
selected pixels are subjected to a voltage pulse of amplitude
V.sub.H +2 .delta.V of duration equal to the switching time while
the nonselected pixels in the selected row and selected columns are
subjected to a voltage pulse of amplitude V.sub.H +.delta.V of
equal duration. The remaining nonselected pixels are held at the
holding voltage. Two-to-one addressing permits selective CLEARING
and WRITING of pixels since .delta.V can be either positive or
negative. However, two-to-one addressing subjects the nonselected
pixels in the selected row and columns to a voltage different from
the holding voltage, i.e., to V.sub.H +.delta.V. Subjecting
nonselected pixels to a voltage different from the holding voltage
is referred to as "cross-talk."
FIG. 7, previously discussed in Example 2, depicts the phenomenon
of cross-talk. That is, FIG. 7 includes switching data for the
three adjacent pixels of the liquid crystal twist cell described in
Example 2, which data depicts what the cross-talk effects of
two-to-one addressing would be, if two-to-one addressing were to be
used in the present invention. In generating the data it was
assumed that two-to-one addressing would be implemented by setting
.delta.V=-V.sub.H /2 (in FIG. 4(b)) to CLEAR selected pixels with a
zero voltage pulse, and by setting .delta.V=+V.sub.H /2 to WRITE
selected pixels with a voltage pulse 2V.sub.H.
As shown in FIG. 7, when a selected pixel, denoted A, is subjected
to a zero voltage pulse of duration equal to about 200 msec in
order to produce a CLEAR transition with two-to-one addressing, the
nonselected pixels in the selected row and columns, denoted B, are
subjected to a voltage pulse of equal duration and of amplitude 0.5
V.sub.H. Similarly, if a selected pixel, denoted D, is subjected to
a voltage pulse of amplitude 2 V.sub.H and a duration equal to
about 68 msec in order to produce a WRITE transition with
two-to-one addressing, the nonselected pixels in the selected row
and columns are subjected to a voltage pulse of amplitude 1.5
V.sub.H of equal duration. As is evident from FIG. 7, the CLEAR
transition curve is relatively shallow near the origin and both
points A and B are close to this curve. If the nonselected pixels B
are to avoid undergoing a CLEAR transition, the duration of the
voltage pulses used to produce CLEAR transitions must be very
precise. Consequently, the performance of the electronics used to
generate two-to-one addressing with the inventive optical display
would have to be very precise, and therefore the electronics would
have to be relatively complex. In addition, and perhaps more
importantly, the widths of the CLEAR and WRITE switching bands
shown in FIG. 7 increase as cell thickness variations increase.
Consequently, for a liquid crystal twist cell which does not have
great thickness uniformity the point B, for example, is likely to
fall within the CLEAR switching band and thus nonselected pixels
will undergo at least partial switching. To avoid this occurrence,
cells addressed with two-to-one addressing would have to be
fabricated with relatively great thickness uniformity, which is
difficult to do. Because there is no cross-talk phenomenon with the
inventive display, the performance of the electronics used in the
invention need not be very precise, and the electronics is
simplified. In addition, the cells included in the inventive
display need not have great thicknes uniformity, making fabrication
of the cells easier.
Two-to-one addressing also subjects nonselected pixels to
cumulative cross-talk effects, i.e., nonselected pixels are
subjected to successive voltage pulses of amplitude different from
the holding voltage during the CLEARING and WRITING of different
selected pixels. This results in random, nonintended switching of
pixels. One procedure for overcoming these cumulative cross-talk
effects is to wait for them to subside. Since there are no such
cumulative effects with, for example, the preferred embodiment of
the inventive display, there is no need to wait for such effects to
subside with the preferred embodiment and thus the preferred
embodiment has a relatively high operating speed.
The phenomenon of cross-talk, associated with the use of two-to-one
addressing, also has the adverse effect of imposing yet further
restrictions on the permissible variation in t/P of the inventive
optical display, making fabrication of the display yet more
difficult. This can be inferred from FIG. 9 which depicts one of
the effects of cross-talk on permissible variations in t/P.
FIG. 9 depicts the typical dependence of relative switching times,
T.sub.S /t.sup.2, on t/P for WRITE transitions and CLEAR
transitions of the liquid crystal twist cell useful in the present
invention. The data displayed in FIG. 9 were obtained by measuring
the switching characteristics of the three adjacent pixels of the
liquid crystal twist cell described in Example 2, at room
temperature (23.5.degree. C.), for different concentrations of CN
in E-7 (and thus different t/P ratios) and for different switching
voltages. FIG. 9 depicts the dependence of T.sub.S /t.sup.2 on t/P
for WRITE switching voltages V.sub.S =2 V.sub.H and 3/2 V.sub.H,
and CLEAR switching voltages V.sub.S =0 and 1/2 V.sub.H. When
two-to-one matrix addressing is used, a switching voltage V.sub.S
=2 V.sub.H (.delta.V=+V.sub.H /2 in FIG. 4(b)) is applied to a
selected pixel in order to produce a WRITE transition, and
cross-talk effects result in nonselected pixels in the selected row
and column being subjected to a switching or cross-talk voltage
V.sub.S =3/2 V.sub.H. Similarly, when a zero switching voltage
(.delta.V=-V.sub.H /2 in FIG. 4(b)) is applied to a selected pixel
to produce a CLEAR transition, cross-talk effects result in
nonselected pixels in the selected row and column being subjected
to a switching or cross-talk voltage V.sub.S =V.sub.H /2. In order
to use two-to-one addressing, the WRITE switching time must be
chosen so as to fall above the V.sub.S /V.sub.H =2 curve but below
the V.sub.S /V.sub.H =3/2 curve (to prevent nonselected pixels from
switching). Similarly, the CLEAR switching time must be chosen so
as to fall above the V.sub.S /V.sub.H =0 curve but below the
V.sub.S /V.sub.H =1/2 curve. While a WRITE switching time can be
chosen without imposing any new limitations on variation in t/P,
any choice of the CLEAR switching time which falls above the
V.sub.S /V.sub.H =0 curve but below the V.sub.S /V.sub.H =1/2 curve
further limits allowable variation in t/P, as is evident from FIG.
9. That is, cross-talk effects inherent in the use of two-to-one
matrix addressing further limit allowable variation in t/P to a
smaller range, for example, than the previously mentioned preferred
range of 0.95 to 1.10 for E-7 doped with CN. Because there are no
cross-talk effects in, for example, the preferred embodiment, as
well as other embodiments, of the inventive optical display, it
follows that the need for great precision in the thickness
uniformity of the liquid crystal twist cell useful in the present
invention is relaxed, resulting in easier fabrication of the
inventive optical display.
Three-to-one addressing does not exhibit the phenomenon of
cross-talk, but two-to-one dose. Therefore, it follows that the
application of the former addressing scheme, rather than the latter
scheme, to the invention optical display reduces the likelihood
that nonselected pixels will undergo a switching transition, for
the reasons discussed above. Consequently, information is more
reliably displayed when three-to-one addressing is applied to the
inventive optical display than when two-to-one addressing is
used.
* * * * *